PS: I want to use a balanced DAC, and that requires a balanced amp.
It's all on AMB's website. Click "Other options" in the M³ project. He explains what needs to be done to get it balanced. You'll need two boards instead of one. You'll have to buy a custom chassis to fit two boards. σ22 is the recommended psu if you're going balanced. The active ground channels can be left unpopulated.
Honestly its going to be A LOT of wasted parts if you try to convert single ended to balanced on the M^3. You might better off starting from scratch. Because you will need an all new case and also you will be wasting all the parts in the active ground channel of the already build board as well as the case. Also I am not sure if you have a walwart with the amp or you used an O22 but you will need an O22 potentially with different parts.
I suggest you sell the current amp and start over on a new balanced M^3. And you should also try building it. It is fun and is also a nice learning experience.
Maybe I'm very tired, and a little pedantic, but are your headphones modified to be balanced? If not, then you don't need a balanced amp. A balanced output (XLR, 1/4" TRS, etc...) can drive an unbalanced input (RCA, 1/8" trs, etc...) just fine. (you can also drive a balanced input with an unbalanced output as well, but that's another subject)
If the only reason you want a balanced amp is because you have a balanced DAC, then just get some XLR to RCA cables and call it a day, or if you want to buy new gear, do as Nikon said and just get some balanced in/unbalanced out transformers to put between your DAC and amp.
And if you want to make your own cable here's the pin out:
2 Center pin
You can contact Ying at YBM and see what he can do for you if you ship your unit back to him in trade for a fully balanced unit (balanced input from the DAC that is, then your choice of how you want the HP connectors ... Ying can give you both balanced and s.e. HP outs, and the balanced can be dual XLR's or 4-pin).
I will stand up for Ying -- a great builder, fantastic communicator, never late -- I was so impressed with his work for me that, when on a biz trip to San Fran, I looked him up and treated him to dinner! And let's have a shout-out for the AMB designs. As Nikongod will attest (he's heard mine), the Beta 22 with Senn 850's is jaw-dropping awesome, and the M^3 with AD2K's, while not quite at the level of the Beta 22/Senn 850 combo, is also amazing.
All built for me with custom mods by YBM. Can't go wrong with the AMB / YBM duo. And a big thanks to Bozebuttons, who several years ago steered me to Beta 22 and YBM -- I was just about to buy a different amp from another builder at the Florida Can Jam when he stopped me. I would have gotten burned (and not just for money ... the other guys's amps actually catch fire).